The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for chemical mechanical polishing.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad can be either a “standard” or a fixed-abrasive pad. A standard polishing pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. Some carrier heads include a flexible membrane that provides a mounting surface for the substrate, and a retaining ring to hold the substrate beneath the mounting surface. Pressurization or evacuation of a chamber behind the flexible membrane controls the load on the substrate. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles, if a standard pad is used, is supplied to the surface of the polishing pad.
The effectiveness of a CMP process can be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad. An uneven load distribution results in a non-uniform material removal and, consequently, in non-uniformity on the surface of the substrate.
A reoccurring problem in CMP is the so-called “edge-effect”, i.e., the tendency of the substrate edge to be polished at a different rate than the substrate center. The edge effect typically results in overpolishing (the removal of too much material from the substrate) at the substrate perimeter, e.g., the outermost five to ten millimeters of a 200 millimeter (mm) wafer. Some methods used to control the pressure applied to the perimeter of substrate do not completely eliminate the edge effect.
Another problem is that engagement of the face of the substrate against the moving polishing pad results in a lateral force applied to the substrate. The lateral force tends to drive the substrate against the retaining ring, deforming the edges and corners of the substrate and creating a non-uniform pressure distribution. It is desirable to reduce the potential range of movement of the substrate and thereby improve the polishing uniformity.
Still another problem relates to difficulties with securing the substrate to the carrier head. Surface tension can cause the substrate to stick to the polishing pad when the carrier head is lifted away from the polishing pad.